Four forces keep an airplane in the sky. They are drag, gravity, thrust and lift.
Weight is the force that pulls airplane towards earth. Airplanes are built in such a manner that their weight is spread from front to back. This keeps the airplane balanced.
Drag is the force that we feel when we walk against a strong wind. Drag slows the airplane. Airplanes are designed to let air pass around them with less drag.
To overcome the forces of drag and gravity, an airplane must generate two forces of its own: thrust and lift.
Thrust is the force that propels an airplane forward on the runway. By Newton's third law — every action has an equal and opposite reaction — the plane's engine generates forward thrust by spewing fuel backwards. Next, as the plane hurtles down the runway, each of its wings slices the air into two streams, one that flows above it and the other, below. The wings are shaped in such a way that the air flowing over them is ultimately deflected downward, and, again because of Newton's third law, the downward motion of the air causes an equal and opposite upward motion of the plane. This is lift.
Every airplane has a specific takeoff speed — the point at which lift overcomes gravity. That critical speed changes based on how much a particular plane weighs. The plane's engine, meanwhile, has to work to provide enough thrust to overcome drag — friction with the air.